The light emission of a light emitting element such as an LED or an LD is a release of energy upon recombination of free electrons and holes, which is induced by the application of a voltage in the forward direction to a pn junction formed in a semiconductor crystal.
The wavelength of the light to be emitted by the light emitting element is mostly decided by the band gap of the material for a semiconductor crystal used as an active layer.
The light is not limited to a single wavelength but may have a specific range of wavelength including a certain central wavelength as a peak.
In the present specification, the central wavelength of the light to be emitted by a light emitting element is referred to as main peak wavelength and the light over a specific range of wavelengths including a certain central wavelength as a peak is referred to as main peak wavelength light.
There is a problematic case in actual light emitting elements, where emission of light occurs in a different wavelength other than the main peak wavelength. Taking the case of a red LED wherein a pn junction of AlGaAs is formed as a light emitting part on a GaAs substrate by crystal growth for example, a light emission having a peak at around 880-900 nm can be observed, in addition to a light emission at the main peak wavelength of 650-670 nm, as shown in FIG. 7.
In the present specification, the central wavelength of such a light emission is referred to as a sub-peak wavelength and the light over a specific range of wavelengths including the sub-peak wavelength as a peak is referred to as sub-peak wavelength light.
The sub-peak wavelength light is not intended in the design of a light emitting element which aims at a light emission at a desired wavelength. The light emitting element involving such an unintended light emission is responsible for the malfunction and various other problems of, for example, a photoelectric sensor wherein the element is used as a light source.
The occurrence of sub-peak wavelength light emission is ascribed to the photoluminescence released as a result of excitation of a crystal substrate due to the irradiation of main peak wavelength light. In view of this phenomenon, the carrier concentration of a substrate is made low to about 1.times.10.sup.16 -1.times.10.sup.18 cm.sup.-3 or a thin film having a similar carrier concentration is formed between the light emitting part and the substrate, in an attempt to suppress the intensity of the photoluminescence which constitutes a sub-peak wavelength light.
Such an attempt is not entirely satisfactory, since the layer with lower carrier concentration suffers from diffusion of dopant from the adjoining light emitting part, etc., causing an increase in carrier concentration. The increased carrier concentration leads to a maximum suppression of the intensity of the sub-peak wavelength light to about 1.5-3% of the intensity of the main peak wavelength light, so that the suppression of the intensity below that level has been unattainable.